Unsteady flow characteristics of particles in plumes are studied with direct simulation Monte-Carlo method (DSMC) in an 8×10^-6 s period. Dissociation model, recombination model and charge exchange model are included based on Bird's chemical reaction models. Molecular hydrogen H₂, atomic hydrogen H, atomic metal X, ion H₂⁺ and ion H⁺ are included. Influence on density distribution of particles in the field due to dissociation and recombination is studied. It shows that the number density of molecular hydrogen H₂ decreases and the number density of atomic H increases since dissociation rate of molecular hydrogen H₂ is faster than recombination of atomic H in the flow field. A charge exchange model is added in dissociation and recombination models. It shows that the number density of ion H₂⁺ decreases and the number density of ion H⁺ increases obviously in charge exchange model.